1) Field of the Invention
The present invention relates to a received signal strength indicator level monitor circuit for a digital mobile communication system. More specifically, the present invention relates to a received signal strength indicator level monitor circuit that can monitor the received signal strength of related channels of a presently receiving channel in other service areas at high-speed.
2) Description of the Related Art
The automobile has become indispensable in contemporary life, and comfort and convenience when driving has become increasingly important. To this end, vehicles can be provided with a mobile communication system which can be used as a telephone system.
An automobile equipped with a mobile communication system, e.g., a mobile telephone system, is called a mobile station, and this mobile station can communicate not only with a fixed telephone such as a telephone in an office or at home but also with another mobile station. This communication is established by radio waves between the mobile station and a land station, and is transferred to a conventional telephone system at the land station.
In a typical analog mobile communication system, a user of the system occupies one frequency for transmission and an another frequency for receiving to communicate. This system is called frequency-division multiple access (FDMA) in a mobile communication system. However, this FDMA is not efficient since one user occupies two frequencies and useable frequencies are decreased as the users of the system increase.
To cope with this inefficiency, a digital mobile communication system is now proposed for practical use. In the digital mobile communication system, a plurality of users, for example, three users occupy one frequency for transmission and another frequency for receiving to communicate by time-sharing. This system is called time-division multiple access (TDMA) in a mobile communication system. To make the explanation brief, a TDMA mobile communication system will be explained hereinafter wherein three users share one frequency. Namely, in the TDMA digital mobile communication system, one frequency is divided into a time unit called a slot, one user can use 1/3 of every slot, and by digital processing, time intervals in every slot for one user are omitted and a telephone call is smoothly continued. Accordingly, from the point of time in the TDMA mobile communication system, one slot is divided into three sections, and 1/3 thereof is used for transmitting a call, 1/3 thereof is used for receiving a call, and 1/3 thereof is empty.
Incidentally, in the mobile communication system, a service area of one land station that is called a zone is limited, and according to the movement of the mobile station from a certain zone to another zone, the mobile station has to change communication channels by changing the communication frequency. Accordingly, in the TDMA digital mobile communication system, when the mobile station is communicating by using one channel at a certain zone, the mobile station intermittently detects the RSSI (received signal strength indicator) levels of the other channels in neighboring zones one by one by using the empty slot. Note that, the useable channels in the neighboring zones are previously determined by the system. The detected RSSI levels of the other channels in the neighboring zones are reported to the land station to which the mobile station is now communicating, and the reported RSSI levels are monitored by the land station. As a result, when the mobile station moves to an other zone, a suitable channel having the highest RSSI level at the other zone is selected by the land station, and the switching of the communication channel is executed.
However, in the TDMA digital mobile communication system, one slot comprises only 20 ms, so that the empty slot for detecting the RSSI level of the other channel has only about 6.7 ms, as a result, high-speed detection of the RSSI level is required.